Colloidal Systems - Surface and Colloid Chemistry: Principles and Applications - page 155

gram of solid increases). This happens because molecules in the bulk phase are
brought to the surface, which needs energy (work). The change in the surface free
energy is the product of the surface area produced (surface area increase) and the
surface tension of the interface created. By changing the surface tension, one may
produce different-sized particles for a given amount of energy input (such as grind-
ing, rolling, or shaking). In the grinding process, an input of mechanical energy is
needed to break down the particles. If surface tension is decreased (by adding a
suitable surface-active agent), then, for the same input of mechanical energy, the
size of particles will decrease with the decrease in surface tension. Moreover, if the
grinding is performed under dry conditions, there will be a strong tendency for the
particles formed to adhere to each other and create the well-known caking problems.
As a related example, the size of crystals decreases when a saturated salt solution
is cooled and a surfactant is added (thus reducing the crystal-solution interfacial
surface tension).
The surface tension of the system can also be changed by grinding with liquid,
thus decreasing interfacial tension. This gives rise to a variety of parameters since,
by adding suitable chemicals (electrolytes or surface-active agents), one can modify
the end-product properties. Conversely, the size of crystals formed from a super-
saturated solution of a substance is related to the surface tension (at the solid-liquid
interface). Thus, to obtain fine crystals, a suitable detergent is added, and thus, finer
crystals are obtained.
A typical example of this is the production of glass fibers that are used for isola-
tion. In order to keep the negatively charged glass fibers from strong adhesion, a cat-
ionic charged surface is sprayed with an active agent, which enhances the isolation
by keeping the fibers from compact structure formation.
In the past few decades, a specific kind of colloidal system based on monodis-
perse size has been developed for various industrial applications. A variety of metal
oxides and hydroxides and polymer lattices have been produced. Monodisperse sys-
tems are obviously preferred since their properties can be easily predicted. On the
other hand, polydisperse systems will exhibit varying characteristics, depending on
the degree of polydispersity.
7.5 aPPlIcatIonS oF colloId SyStemS
The stability of colloid suspensions is an important criteria in the manufacture of a
large number of industrial products where these are the basic building blocks (food
colloids, pollution control, emulsions, wastewater treatment).
An example of a food colloid is mayonnaise (a mixture of vegetable oil plus egg yolk
and vinegar which is an emulsion of oil in water).
The electrostatic forces in many systems play a dominant role, such as the separa-
tion process (filtration) in wastewater treatment.
7.5.1 W a S T e W a T e r T r e a T m e n T a n d c o n T r o l (z e T a p o T e n T I a l )
Wastewater contains different kinds of pollutants (dissolved substances, suspended
particles), and is treated in suitable plants before the processed water is released.